In the manufacture of plastic articles from thermoplastic materials, such as polyethylene terephthalate, polypropylene, acrylonitrils and the like, it has been found that articles of enhanced physical properties are formed by stretching and blowing a preform into the final container. While it is possible, with some materials, to injection mold or extrusion form a parison which is then stretched and blown while still thermoplastic from residual heat, it is preferable, with other materials, to reheat a preformed parison to the exact temperature desired and then stretch and blow the reheated parison to its final configuration.
Various processes and apparatus have been developed to carry out such blow molding operations. In one such process, the parison is reheated in an oven through which the parison travels continuously or intermittently in a heated atmosphere. The parisons are removed from the oven, either singly or in groups, and is stretched and blown. Alternatively, a series of parisons is placed on a heating wheel and conveyed past an array of fixed heating elements surrounding the periphery of the wheel until the parison is heated to the desired temperature. The parisons are then transferred to a second blow molding wheel where they are blown in succession in a series of molds traveling with the wheel.
In either of these prior art processes and apparatus, the handling of the parisons to the oven or the heating wheel, the transfer of the parisons after heating to the blow molding apparatus, and the blow molding of the parisons requires a complex, expensive apparatus having multiple blow molds and requiring large amounts of radiant energy to heat the moving parisons. Further, it is difficult to selectively heat different portions of the parison to facilitate the blowing of difficult shapes.